It is well recognized in the electronics industry that electrostatic discharge ("ESD") can severely damage integrated circuit devices. While electrical discharges as low as 30 volts can damage semiconductor integrated circuit ("IC") devices, electrostatic discharges of several hundred volts can occur during the routine handling of printed circuit boards to which IC devices are attached.
In recognition of the potential for ESD voltages damaging sensitive semiconductor devices, many ICs are provided with on-chip ESD protection circuitry adapted to prevent ESD voltages from damaging other on-chip circuitry. Typically, such circuitry is provided for each individual pin of the integrated circuit. Those of ordinary skill in the art will be familiar with a great many different types and implementations of on-chip ESD protection circuitry. A common approach is to provide circuitry responsive to voltages above a certain threshold to divert such voltages directly to a ground terminal, thereby avoiding other circuitry on the IC. An example of this is proposed in U.S. Pat. No. 4,692,781.
Another common method of avoiding ESD damage to semiconductor devices is to add external (i.e., off-chip) ESD protection circuitry to the printed circuit boards upon which one or more ICs are disposed. Such external ESD protection circuitry is designed to divert any electrostatic discharge away from operational circuitry and toward a ground terminal. Such external ESD protection circuitry is typically made up of discrete components attached to the printed circuit board.
There are perceive disadvantages to the aforementioned methodology for avoiding ESD damage to ICs mounted on printed circuit boards. First, the discrete components that make up the ESD protection circuitry occupy space on the printed circuit board, thereby reducing the amount of space available for operational circuitry. Second, designing an ESD protection circuit for a printed circuit board increases the cost of designing that printed circuit board. Finally, such external ESD protection circuitry may not provide any ESD protection when the printed circuit board is not attached to a solid ground line. For example, a printed circuit board is not attached to a ground line when it is being shipped from the manufacturer to a customer, and is therefore vulnerable during shipping to ESD-related damage.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the foregoing perceived disadvantages.